Among known compounds having an indolo[2,3-b]quinoxaline ring with an antitumor activity are two kinds of the compounds reported in Japanese Patent Laid-open No. 63-502587 and ibid. 63-502588.
However, the antitumor effect of these compounds is insufficient.
Accordingly, an object of the present invention is to provide novel indolo[2,3-b]quinoxaline derivatives having an excellent antitumor activity.